Functional anatomy of the ciliary muscle in birds and humans

Abstract

The functional anatomy of the avian ciliary muscle has been difficult to describe due to its inaccessible location inside the eye. This thesis compares the ciliary muscle of birds and humans in both the relaxed and contracted states. The two groups both accommodate by changing the shape of the lens, although the ciliary body and ciliary muscle morphology are very different.

The ciliary muscles of the chicken (Gallus gallus domesticus), the pigeon (Columbia livia), the kestrel (Falco sparverius), and the hooded merganser (Marcus cucullatus) (four species with differing accommodative needs) were examined histologically. The ciliary muscle of all four species can be divided into three main muscle fibre groups based on insertion and origin: anterior, posterior and internal. The anterior muscle fibre group originates at the sclera under the scleral ossicles and inserts into the inner lamellae of the cornea. During accommodation these fibres pull the cornea posteriorly, changing the curvature of the cornea. The posterior muscle fibre group originates on the sclera and inserts posteriorly onto the baseplate of the ciliary body. The posterior fibre group acts on the baseplate of the ciliary body, pulling it forward to change the curvature of the lens during accommodation. The internal muscle fibre group extends from the baseplate of the ciliary body to the inner lamellae of the cornea and thus has a role in both corneal and lenticular accommodation. Species differences do exist as seen in subgroups determined by the orientation of the fibres between the relaxed and contracted states and the percentage of fibres within the main muscle fibre group. In general, the majority of ciliary muscle fibres in the chicken, pigeon, and kestrel are in the anterior muscle fibre group, suggesting an emphasis on corneal accommodation; in the ciliary muscle of the hooded merganser, the majority of fibres are in the internal and posterior muscle fibre groups, indicating that lenticular accommodation is the predominant form of accommodation.

For the study of human ciliary muscle, fifteen pairs of eyes ranging in age from 0 to 107 years were treated with 20% pilocarpine and 5% atropine. The ciliary muscle decreases in length (8-20%) and in width (5-14%). Changes with contraction were not found in the other measurements. The fibres of the ciliary muscle releases zonular tension by shortening, but not by moving forward. With age the ciliary muscle shortens, widens, and moves forward. Changes in ciliary muscle dimensions were seen at all ages, indicating no loss of muscle contractility. The amount of connective tissue in the ciliary muscle increases (24%) with age. The aging ciliary muscle is able to contract but the reduction of zonular tension brought on by age, may prevent lenticular changes, thereby contributing to presbyopia.